Surface acoustic wave device and diamond base material for the same

ABSTRACT

A diamond base material for surface acoustic wave device, which includes: a low-resistivity base material, and a high-resistivity diamond layer having a thickness of 5-50 μm disposed on the low-resistivity base material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diamond base material which maysuitably be used for fabricating a surface acoustic wave device(hereinafter, mainly referred to as "SAW device"), and a SAW deviceutilizing such a diamond base material.

2. Related Background Art

Diamond has a highest acoustic propagation velocity among all of thematerials which are present on the earth, and has a band gap energy of5.5 eV, which is very large as compared with those of knownsemiconductor substances. In addition, diamond is transparent withrespect to light having a wavelength ranging from a near-ultravioletregion to a near-infrared region. Accordingly, it is expected thatdiamond can improve an operating characteristic and/or can widen anoperating range or latitude in various technical or scientific fieldssuch as acoustic, surface acoustic wave, optical, and semiconductorfields.

In addition, since diamond has a good thermal conductivity, not onlydiamond itself may desirably function as a heat sink material, but alsoa stable functional device or element (such as optical functionaldevice) which requires substantially no temperature compensation forheat may be constituted by use of diamond.

By utilizing such dynamical, electric or electronic characteristics,diamond has been used for a wide variety of dynamical, optical, electricor electronic devices (as described in, e.g., "Diamond Thin Film",written by Tadao Inuzuka, pp. 99-115, 1990, published by KyoritsuShuppan (Tokyo, JAPAN)). Specific examples of such a device utilizingdiamond may include a SAW device which may suitably be used for ahigh-frequency filter, etc.

Heretofore, as the above-mentioned SAW device, there is known one havinga multilayered structure comprising a diamond thin film and acombination of an interdigital transducer or electrode having acomb-like shape (hereinafter, mainly referred to as "IDT") and apiezoelectric substance disposed on the diamond thin film, as disclosedin Japanese Patent Publication (KOKOKU) No. 38874/1979 (i.e., Sho54-38874) and Japanese Laid-Open Patent Application (KOKAI) No.62911/1989 (i.e., Sho 64-62911).

As described above, the SAW device has a structure comprising a diamondthin film and a piezoelectric layer disposed thereon, wherein thepiezoelectric layer comprises a piezoelectric substance such as ZnO,quartz crystal, LiNbO₃ and LiTaO₃. Accordingly, the above-mentioned IDTconstituting the SAW device is usually grounded by using a wire bondingmethod, i.e., a method wherein the IDT is grounded by use of a wirewhich is electrically connected to the IDT by way of a bonding pad.

However, the above-mentioned conventional wire bonding method has thefollowing drawbacks.

(1) In the wire bonding method, it is necessary to provide, on a deviceto be wire-bonded, a sufficient space corresponding to the area ofbonding pads to be used for the wire bonding, and therefore the size ofthe device itself necessarily becomes large to a considerable extent.

(2) In a case where the SAW device has a multi-electrode structurecomprising a plurality of electrodes (inclusive of IDTs), the number ofthe bonding wires and the number of the bonding pads become considerablylarge, and therefore the size of the device itself becomes large to afurther extent. In addition, the location or position of the bondingpads is considerably limited so that the bonding wires may be connectedto the resultant bonding pads.

(3) In a case where the SAW device has a multi-electrode structure, thebonding steps themselves for conducting the wire bonding becomecomplicated and troublesome.

Particularly, in recent years, there has strongly been demanded asmall-sized SAW device of a surface-mounting type, i.e., a SAW devicewhich may be mounted to a surface or surface portion of an apparatuswherein the SAW device is to be utilized. Accordingly, there has beendesired the development of a grounding method which is suitablyapplicable to such a small-sized SAW device of the surface-mountingtype.

SUMMARY OF THE INVENTION

An object of the present invention Is to provide a diamond base materialor SAW device which has solved the above-mentioned problems encounteredin the prior art.

Another object of the present invention is to provide a diamond basematerial or SAW device which may attain easy grounding of an electrodeto be disposed on the diamond base material or to be disposed in the SAWdevice.

As a result of earnest study, the present inventors have found that itis very effective to use a low-resistivity member itself constituting adiamond/low-resistivity member structure, as a member for groundingconnection.

The diamond base material according to the present invention is based onthe above discovery and comprises: a low-resistivity base material, anda high-resistivity diamond layer having a thickness of 5-50 μm disposedon the low-resistivity base material.

The present invention also provides a diamond base material for surfaceacoustic wave device, comprising: a base material, a low-resistivitydiamond layer having a thickness of 0.3-10 μm disposed on the basematerial, and a high-resistivity diamond layer having a thickness of5-50 μm disposed on the low-resistivity diamond layer.

The present invention further provides a surface acoustic wave device,comprising:

a low-resistivity base material;

a diamond layer disposed on the low-resistivity base material, thediamond layer having a through hole formed therein;

a piezoelectric substance laver disposed on the diamond layer; and

an interdigital transducer disposed in contact with the piezoelectricsubstance layer, the interdigital transducer being connected to thelow-resistivity base material for grounding by way of the through holeformed in the diamond layer.

The present invention further provides a surface acoustic wave device,comprising:

a base material;

a low-resistivity diamond layer disposed on the base material;

a high-resistivity diamond layer disposed on the low-resistivity diamondlayer, the high-resistivity diamond layer having a through hole formedtherein;

a piezoelectric substance layer disposed on the high-resistivity diamondlayer; and

an interdigital transducer disposed in contact with the piezoelectricsubstance layer, the interdigital transducer being connected to thelow-resistivity diamond layer and/or base material for grounding by wayof the through hole formed in the high-resistivity diamond layer.

The present invention further provides a process for producing a surfaceacoustic wave device comprising: a low-resistivity base material; adiamond layer disposed on the low-resistivity base material, the diamondlayer having a through hole formed therein; a piezoelectric substancelayer disposed on the diamond layer; and an interdigital transducerdisposed in contact with the piezoelectric substance layer, theinterdigital transducer being connected to the low-resistivity basematerial for grounding by way of the through hole formed in the diamondlayer;

wherein the through hole is formed by use of a dry etching method.

The present invention further provides a process for producing a surfaceacoustic wave device, comprising: a base material; a low-resistivitydiamond layer disposed on the base material; a high-resistivity diamondlayer disposed on the low-resistivity diamond layer, thehigh-resistivity diamond layer having a through hole formed therein; apiezoelectric substance layer disposed on the high-resistivity diamondlayer; and an interdigital transducer disposed in contact with thepiezoelectric substance layer, the interdigital transducer beingconnected to the low-resistivity diamond layer and/or base material forgrounding by way of the through hole formed in the high-resistivitydiamond layer;

wherein the through hole is formed by use of a dry etching method.

In the present invention, the above-mentioned IDT may be connected toeither one of or both of the low-resistivity base material (orsubstrate) and the low-resistivity diamond layer, for the purpose ofgrounding.

According to the present invention, it is possible to extend thelatitude or degree of freedom in the electrode connection or electricwiring to be utilized in a diamond base material having a structure ofhigh-resistivity diamond/low-resistivity portion. As a result, it ispossible to remarkably extend the latitude or freedom in the design orplan, in a case where various kinds of devices are constituted by use ofthe above-mentioned diamond base material. For example, in the presentinvention, the bonding pad to be used for wire bonding at the time ofthe actual SAW device mounting or packaging is omissible, and thereforethe size of the SAW device itself may be reduced. In addition, in thepresent invention, since the condition or requirement for the selectionof grounding position or location may relaxed or relieved, the latitudeor degree of freedom in the device design may be extended. From such aviewpoint, the present invention may also contribute to the reduction inthe device size.

Particularly, in a case where a SAW device having a multi-electrodestructure is to be constituted, the effect of reducing the device sizebecomes considerably marked.

Furthermore, since the bonding pad For grounding is omissible in thepresent invention, at the time of the device mounting, the number of thewire bonding steps may be minimized so as to wire-bond a small number oflines for minimum requirement (e.g., it may be sufficient to conduct thewire bonding step only for a signal line for minimum requirement). As aresult, the wire bonding steps per se may also be simplified.

Accordingly, the present invention may remarkably facilitate thefabrication or manufacturing of a small-sized device of asurface-mounting type, on the basis of a feature, or a combination offeatures as described above.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an embodiment of thediamond base material according to the present Invention.

FIG. 2 is a schematic cross-sectional view showing another embodiment ofthe diamond base material according to the present invention.

FIG. 3 is a schematic cross-sectional view showing an embodiment of thelayer structure of a SAW device according to the present invention.

FIG. 4 is a schematic plan view showing an embodiment of the structureof an IDT which is usable in the SAW device according to the presentinvention.

FIG. 5 is a schematic plan view showing another embodiment of thestructure of an IDT which is usable in the SAW device according to thepresent invention.

FIG. 6 is a schematic plan view showing a further embodiment of thestructure (multi-electrode structure) of an IDT which is usable in theSAW device according to the present invention.

FIG. 7 is a schematic plan view showing a further embodiment of thestructure (multi-electrode structure) of an IDT which is usable in theSAW device according to the present invention.

FIG. 8 is a schematic plan view showing a further embodiment of thestructure (multi-electrode structure) of an IDT which is usable in theSAW device according to the present invention.

FIG. 9 is a schematic cross-sectional view showing another embodiment ofthe layer structure of a SAW device according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinbelow, the present invention will be described in detail withreference to the accompanying drawings, as desired.

(Base material)

In the present invention, as a base material (or substrate), diamondand/or another material (such as silicon) may be used without particularlimitation, as long as such a base material may provide a surface onwhich a high-resistivity diamond layer or a low-resistivity diamondlayer may be disposed.

In a case where the above base material is a low-resistivity basematerial, the volume resistivity thereof may preferably be 10³ Ω·cm orlower.

(Diamond)

When the diamond constituting the base material or SAW device accordingto the present invention comprises a diamond layer (or diamond film),the method or process for growing the diamond layer is not particularlylimited. More specifically, the growth process to be used in the presentinvention may be selected from known processes without particularlimitation. Specific examples of such known processes may include: a CVD(Chemical Vapor Deposition) method, a microwave plasma CVD method, a PVD(Physical Vapor Deposition) method, a sputtering method, an ion platingmethod, a plasma jet method, a flame method, a hot filament method, etc.

In a case where the above diamond layer is a low-resistivity diamondlayer, the volume resistivity thereof may preferably be 10³ Ω·cm orlower. On the other hand, in a case where the above diamond layer is ahigh-resistivity diamond layer, the volume resistivity thereof maypreferably be 10⁶ Ω·cm or higher.

The above high-resistivity diamond layer may preferably have a thicknessin the range of 5-50 μm. On the other hand, the above low-resistivitydiamond layer may preferably have a thickness in the range of 0.3-10 μm.

The ratio between the volume resistivities of the above-describedhigh-resistivity diamond layer and low-resistivity diamond layer (orlow-resistivity base material) may preferably be not less than 10³, morepreferably, not less than 10⁵ (particularly preferably, not less than10⁶) in view of retention of a good balance between insulating propertyand conductivity.

In a case where the diamond base material according to the presentinvention having the above-described layered structure is used for abase material for a SAW device, the surface of a diamond layer (e.g., ahigh-resistivity diamond layer) on which an electrode comprising the SAWdevice (such as IDT and short-circuiting electrode) is to be disposed,may preferably have a surface roughness of 300 angstrom or smaller (morepreferably, 150 angstrom or smaller) in terms of average roughness(R_(a)) measured by a probe method (i.e., a method using a needle as aprobe or tracer) according to Japanese Industrial Standard (JIS) B0601-1970. With respect to the details of the average roughness R_(a), abook entitled "KIKAI KEISOKU (Mechanical Metrology)", 77-80 pages, 1974,written by Osamu Taniguchi, published by Yokendo K. K. (Tokyo, JAPAN)may be referred to.

(Embodiments of layer structure)

Preferred examples of the Layer structure to be used in the presentinvention are shown in schematic cross-sectional views of FIGS. 1 and 2.In the embodiment as shown in FIG. 1, a high-resistivity diamond layer 2is disposed on a low-resistivity silicon (Si) substrate. On the otherhand, in the embodiment as shown in FIG. 2, a low-resistivity diamondlayer 3 is disposed on a silicon substrate 1a, and further ahigh-resistivity diamond layer 2 is disposed on the abovelow-resistivity diamond layer 3. In a case where Si is used as a basematerial or substrate, it becomes easy to form a diamond base material(or a SAW device utilizing the base material) on the same substrate onwhich another device is to be disposed.

(SAW device)

In the present invention, a SAW device comprises: a low-resistivitymember (such as low-resistivity substrate and low resistivity diamondlayer), a diamond layer disposed on the low-resistivity member, apiezoelectric substance layer disposed on the diamond layer, and an IDTdisposed in contact with the piezoelectric substance layer. In this SAWdevice, the above-described IDT is connected to the low-resistivitymember (either one or both of the low-resistivity base material and thelow-resistivity diamond layer) by way of (or via) a through hole for thepurpose of grounding.

(Through hole)

In a case where a SAW device is constituted by using a diamond basematerial according to the present invention having the above-describedlayer structure, a through hole is formed in the above diamond layer(e.g., the high-resistivity diamond layer). The shape (planer shape,sectional shape, three-dimensional shape, etc.) and/or the size of thethrough hole is not particularly limited, as long as an electricalconnection may be attained by way of (or via) such a through holebetween an electrode (such as IDT and short-circuiting electrode)constituting a SAW device, and a low-resistivity member (such aslow-resistivity diamond layer and low-resistivity base material) to bedisposed under the high-resistivity diamond layer.

Specific examples of the method for forming such a hole in diamond mayinclude two methods, i.e., a method using laser such as YAG laser andexcimer laser, and a method using dry etching. The above-mentionedthrough hole may preferably be formed by use of a dry etching method inview of the controllability, the size of a through hole to be formed,the processability of a through hole for finishing thereof, etc. As sucha dry etching method, any one or combination selected from known dryetching methods such as gas etching, plasma etching, reactive ionetching (RIE), and ion milling may be used without particularlimitation. Among these, RIE may preferably be used in view of theselectivity of a material to be etched, and/or etching anisotropy.

(Embodiments of SAW device)

FIG. 3 shows a schematic cross-sectional view of an embodiment of theSAW device according to the present invention.

Referring to FIG.3, in this embodiment, the SAW device comprises: a basematerial 1a, a low-resistivity diamond layer 3 disposed on the basematerial 1a, a high-resistivity diamond layer 2 disposed on thelow-resistivity diamond layer 3, a piezoelectric substance layer 4disposed on the high-resistivity diamond layer 2, and an IDT 5 disposedin contact with the piezoelectric substance layer 4. In thehigh-resistivity diamond layer 2, a through hole 6 is formed so as toallow electric connection between the IDT 5 and the low-resistivitydiamond layer 3.

FIG. 9 shows another embodiment of the SAW device according to thepresent invention. Referring to FIG.9,. in this embodiment, the SAWdevice comprises: a low-resistivity base material 1, a high-resistivitydiamond layer 2 disposed on the low-resistivity base material 1, apiezoelectric substance layer 4 disposed on the high-resistivity diamondlayer 2, and an IDT 5 disposed in contact with the piezoelectricsubstance layer 4. In the high-resistivity diamond layer 2, a throughhole 6 is formed so as to allow electric connection between the IDT 5and the low-resistivity base material 1.

As described above, when an electrode is grounded via a through hole 6,the freedom or latitude in the device design may be widened. Inaddition, in such a case, it becomes possible to further miniaturize adevice ,e.g., by decreasing the number of wires to be used for wirebonding. The member or place to be connected with such an electrode forthe purpose of grounding may be either one or both of theabove-described low-resistivity base material 1 and low-resistivitydiamond layer 3.

In the embodiment as described above with respect to FIG. 3 (or FIG. 9),the IDT 5 is disposed under the lower surface of the piezoelectricsubstance layer 4 (i.e., between the piezoelectric substance layer 4 andthe high-resistivity diamond layer 2). However, the IDT 5 may bedisposed in another position (e.g., on the upper surface of thepiezoelectric substance layer 4), as long as the IDT 5 is disposed incontact with the piezoelectric substance layer 4. (Piezoelectricsubstance layer)

In the SAW device according to the present invention, the piezoelectricsubstance constituting the piezoelectric substance layer 4 may compriseany of known piezoelectric substances such as ZnO, quartz, LiNbO₃, andLiTaO₃ without particular limitation. The thickness of the piezoelectricsubstance layer may be appropriately selected in accordance with thekind of the piezoelectric substance to be used and/or the characteristicof a SAW device to be fabricated, etc.

The method for forming a film or layer of the above-mentionedpiezoelectric substance is not particularly limited. More specifically,such a film forming method may be any one or combination selected fromknown methods such as CVD (Chemical vapor deposition) method, microwaveplasma CVD method, PVD (Physical vapor deposition) method, sputteringmethod, and ion plating method without particular limitation. In view ofthe uniformity, mass productivity and piezoelectric property of theresultant film or layer, a sputtering method (particularly, an RF (radiofrequency) magnetron sputtering method) may particularly preferably beused. (IDT)

The material constituting an IDT to be used in the present invention isnot particularly limited, as long as it is an electrically conductivematerial. In view of the conversion to bulk wave, Al (aluminum) mayparticularly preferably be used for the above material for IDT.

The thickness of the IDT is not particularly limited, as long as it mayfunction as such an electrode. The IDT may preferably have a thicknessin the range of about 100-3000 angstrom (more preferably, about 300-1500angstrom).

The planar shape of the IDT is not particularly limited as long as itfunctions as such an electrode. Preferred examples of such an electrodemay include a so-called "single-type electrode" as shown in theschematic plan view of FIG. 4, and a so-called "double-type electrode"as shown in the schematic plan view of FIG.5.

In the present invention, the above-described IDT may constitute amulti-electrode structure as shown in the schematic plan views of FIGS.6 to 8. With respect to the details of such a multi-electrode structure,papers entitled "Densi Joho Tsushin Gakkai Gijutu Kenkyu Hokoku(Technical Research ReDort published by Electronics Information andCommunication Society)", US-81, 25-31 pages (1990); and "FUJITSU"(Technical Bulletin), vol. 43(2), 179-185 pages (1992) may be referredto.

(Short-circuiting electrode)

In the SAW device according to the present invention, a short-circuitingelectrode may further be disposed as desired. Such a short-circuitingelectrode is an electrode which has a function of providing anequipotential state in an electric field so as to change the SAW(surface acoustic wave) characteristic of the SAW device. In general,the short-circuiting electrode (not shown) may be disposed on a surfaceof a piezoelectric substance layer 4 which is opposed to the othersurface of the piezoelectric substance layer 4 on which an IDT 5 isdisposed (i.e., the surface opposed to the IDT 5 through the medium ofthe piezoelectric substance layer 4; e.g., the upper surface of thepiezoelectric substance layer 4 in the embodiment shown in FIG. 3).

The short-circuiting electrode may preferably comprise a (thin) film ofa metal such as Al, Au, and Al--Cu. Since the short-circuiting electrodehas a different function from that of the IDT, the material constitutingthe short-circuiting electrode is not necessarily the same as thematerial constituting the IDT.

The thickness of the short-circuiting electrode is not particularlylimited as long as it functions as such an electrode. In general, theshort-circuiting electrode may preferably have a thickness in the rangeof about 1-100 μm (more preferably, about 5-30 μm).

For example, the short-circuiting electrode may preferably have a planarshape of a "solid electrode" or "non-patterned electrode" which has thesame occupation area as that of the IDT.

Hereinbelow, the present invention will be described in more detail withreference to specific Examples.

EXAMPLE 1

A 25 μm-thick high-resistivity polycrystalline diamond layer having avolume resistivity 10⁷ Ω·cm was formed on a low-resistivity siliconsubstrate having a volume resistivity of 10⁻¹ Ω·cm, under the followingconditions.

<Microwave plasma CVD method>

Microwave power: 500 W

Pressure: 40 Torr

Reactant gas: CH₄ /H₂ =3/100

Substrate temperature: 950° C.

A portion (i.e., a though hole having a diameter of 30 μm) through whichgrounding was to be effected, was formed in the above-describedhigh-resistivity polycrystalline diamond layer by using a dry etchingprocess and a metal (film) formation process, under the followingconditions.

<Dry etching conditions>

Gas: O₂ /Ar=1/100

Power: 500 W

Pressure: 70 Torr

<Metal formation conditions>

(First step)

Ti/Pt/Au sputtering (thickness: 5000 angstrom)

(Second step)

Au plating (thickness: 20 μm)

Then, on the resultant substrate having a high-resistivity diamond/Sistructure in which the above through hole had been formed, an IDTcomprising Al (thickness: 700 angstrom, planar shape: one as shown inFIG. 8) was formed by using a sputtering method as an ordinary Al filmformation process. At the time of the formation of the above IDT, theelectrical connection between a grounding part of the IDT and theabove-described low-resistivity silicon substrate was achieved by thedeposition of Al onto (or into) the above-described through hole.

On the resultant diamond substrate in which the grounding part of theIDT had been electrically connected to the low-resistivity siliconsubstrate as described above, a film of ZnO as a piezoelectric substance(thickness: 5000 angstrom) was formed by using a sputtering methodthereby to fabricate a diamond surface acoustic wave device (SAW device)having a structure as shown in FIG. 9.

The thus obtained surface acoustic wave device had a multi-electrodestructure, and the number of parts or points of the multi-electrodestructure to be grounded was 15 (fifteen). Accordingly, the chip area ofthe resultant device was reduced to about 2/3as compared with that of achip which had been fabricated by using an ordinary wire bonding method,on the basis of the omission of bonding pad portions, etc.

EXAMPLE 2

On a silicon substrate, a 5 μm-thick low-resistivity polycrystallinediamond layer having a volume resistivity 10⁻² Ω·cm was formed under thefollowing conditions. Then, on the resultant low-resistivitypolycrystalline diamond layer, a 20 μm-thick high-resistivitypolycrystalline diamond layer having a volume resistivity 10⁹ Ω·cm wasformed under the following conditions.

<Conditions for formation of low-resistivity polycrystalline diamondlayer>

(Microwave plasma CVD method)

Microwave power: 500 W

Pressure: 40 Torr

Reactant gas: CH₄ /H₂ =3/100

B₂ H₆ /CH₄ =1000 ppm

Substrate temperature: 950° C.

<Conditions for formation of high-resistivity polycrystalline diamondlayer>

(Microwave plasma CVD method)

Microwave power: 500 W

Pressure: 40 Torr

Reactant gas: CH₄ /H₂ =3/100

Substrate temperature: 950° C.

A portion (i.e., a though hole having a diameter of 25 μm) through whichgrounding was to be effected, was formed in the above-describedhigh-resistivity polycrystalline diamond layer by using a dry etchingprocess and a metal (film) formation process under the followingconditions.

<Dry etching conditions>

Gas: O₂ /Ar=1/100

Power: 500 W

Pressure: 70 Torr

<Metal formation conditions>

(First step)

Al sputtering (thickness: 5000 angstrom)

(Second step)

Al plating (thickness: 20 μm)

Then, on the resultant substrate having a high-resistivitydiamond/low-resistivity diamond/Si structure in which the above throughhole had been formed, an IDT comprising Al (thickness: 700 angstrom,planar shape: one as shown in FIG.8) was formed by using a sputteringmethod as an ordinary Al film formation process. At the time of theformation of the above IDT, the electrical connection between agrounding part of the IDT and the above-described low-resistivitysilicon substrate (or low-resistivity diamond layer) was achieved by thedeposition of Al onto (or into) the above-described through hole.

On the resultant diamond substrate in which the grounding part of theIDT had been electrically connected to the low-resistivity siliconsubstrate (or low-resistivity diamond layer) as described above, a filmof ZnO as a piezoelectric substance (thickness: 5000 angstrom) wasformed by using a sputtering method thereby to fabricate a diamondsurface acoustic wave device (SAW device) having a structure as shown inFIG.3.

The thus obtained surface acoustic wave device had a multi-electrodestructure, and the number of parts or points of the multi-electrodestructure to be grounded was 15 (fifteen). Accordingly, the chip area ofthe resultant device was reduced to about 2/3as compared with that of achip which had been fabricated by using an ordinary wire bonding method,on the basis of the omission of bonding pad portions, etc.

As described hereinabove, according to the present invention, there isprovided a diamond base material for surface acoustic wave device, whichcomprises: a low-resistivity member (a low-resistivity base material,low-resistivity diamond layer, etc.), and a high-resistivity diamondlayer disposed on the low-resistivity member.

The present invention also provides a SAW device comprising theabove-mentioned base material as a component, wherein a through hole hasbeen formed in the above high-resistivity diamond layer for the purposeof grounding

According to the present invention, at least a portion of bonding padsfor wire bonding which have heretofore been used at the time of devicemounting may be omitted, and therefore the size of the resultant devicemay be reduced. In the case of a device having a multi-electrodestructure, the effect of reducing the size of the device per se becomesparticularly marked.

In addition, the condition or requirement for the selection of agrounding position or location may be relaxed or relieved, and thereforethe degree of freedom or latitude in device design may be increased. Asa result, the size of a device may also be reduced from such aviewpoint.

In addition, according to the present invention, the number of the wirebonding steps may be minimized (e.g., it mav be sufficient to conductthe wire bonding step only for a signal line for minimum requirement).Therefore, the wire bonding steps itself may also be simplified.

As a result, the present invention may remarkably facilitate thefabrication or manufacturing of a small-sized device of asurface-mounting type, on the basis of a feature, or a combination offeatures as described above.

The basic Japanese Application No. 137628/1994 filed on Jun. 20, 1994(i.e., Hei 6-137628) is hereby incorporated by reference.

Many modifications of the present invention may be made withoutdeparting from the essential scope thereof. It should be understood thatthe present invention is not limited to the specific embodiments asdescribed.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A diamond base material for surface acoustic wavedevice, comprising: a low-resistivity base material, and ahigh-resistivity diamond layer having a thickness of 5-50 μm disposed onthe low-resistivity base material.
 2. A diamond base material forsurface acoustic wave device according to claim 1, wherein the ratio ofthe resistivities between the high-resistivity diamond layer and thelow-resistivity base material is not less than 10³.
 3. A diamond basematerial for surface acoustic wave device, comprising: a base material,a low-resistivity diamond layer having a thickness of 0.3-10 μm disposedon the base material, and a high-resistivity diamond layer having athickness of 5-50 μm disposed on the low-resistivity diamond layer.
 4. Adiamond base material for surface acoustic wave device according toclaim 3, wherein the ratio of the resistivities between thehigh-resistivity diamond layer and the low-resistivity diamond layer isnot less than 10³.